fix warning:

equality comparison with extraneous parentheses

contrib/unzip/unzip.c

@@ -1246,7 +1246,7 @@ extern int ZEXPORT unzReadCurrentFile  (file, buf, len)
         return UNZ_PARAMERROR;
 
 
-    if ((pfile_in_zip_read_info->read_buffer == NULL))
+    if (pfile_in_zip_read_info->read_buffer == NULL)
         return UNZ_END_OF_LIST_OF_FILE;
     if (len==0)
         return 0;